The present invention relates to a process for preparing core-shell aqueous dispersions having the core based on PTFE and shell on polystyrene (PS), wherein the particles contain an amount by weight of PTFE from 3 to 50%, preferably from 5 to 30%, and to the core-shell product as above defined.
In particular the invention relates to a process wherein, starting from PTFE latexes containing particles having a diameter from 5 to 100 nm, preferably from 10 to 70 nm, on which styrene is polymerized, with formation of a PS shell, a latex containing particles formed by a PTFE core and by a polystyrene shell is obtained.
The so obtained nanocomposite can be used as such to obtain manufactured articles or coatings having improved mechanical properties, resistance to wear. The nanocomposites of the invention can also be used as additives for polymers styrene-based, due to their very good compatibility with said polymers.
In U.S. Pat. No. 4,334,037 it is known to obtain manufactured articles by moulding of compositions obtained by mixing PS granules with at least 5% by weight of PTFE granules having an average particle diameter lower than 50 xcexcm, preferably having a diameter from 5,000 to 7,000 nm. The PTFE granules were uniformly dispersed in the polystyrene granules, the mixture heated to form a homogeneous plasticized mass, that was afterwards extruded. The manufactured articles obtained by moulding the extruded material show good resistance to wear and to friction. Said manufactured articles are particulary suitable to be used in aqueous medium. The drawback of said manufactured articles is that they do not show homogeneous properties when said articles are obtained by direct moulding the mixture of granules of PTFE with polystyrene. Tests carried out by the Applicant have shown that the two polymers are poorly compatible and manufactured articles having poor mechanical properties, such as for example the impact-resistance, are obtained.
Besides tests carried out by the Applicant to try to increase the compatibility between PTFE and PS starting from their polymer latexes, have not lead to a meaningful improvement with respect to the powder mixing. Also in this case segregation phenomena take place of the two polymers into separated phases, during the coagulation. This last step is essential to obtain the material which is then subjected to moulding or to extrusion to obtain the final manufactured article. Greater segregation is obtained and therefore lower mechanical properties, starting from PTFE latexes wherein the particles have a diameter lower than 100 nm.
The need was felt to have available compositions styrene-based polymers containing PTFE having improved mechanical properties, improved wear resistance without showing the drawbacks of the compositions of the prior art.
An object of the present invention are compositions containing core-shell particles having a core polytetrafluoroethylene-based polymers (PTFE) and shell styrene-based polymers (PS), wherein the particles contain an amount by weight of PTFE from 3 to 50%, preferably from 5 to 30% by weight, wherein the PTFE core particles have an average diameter from 5 to 100 nm, preferably from 10 to 70 nm, the core-shell particles having an average diameter from 10 to 170 nm, preferably from 20 to 100 nm.
For polytetrafluoroethylene-based polymers (PTFE), homopolymers of tetrafluoroethylene or copolymers of TFE with one or more monomers containing at least one ethylene type unsaturation are meant, said comonomers in amounts from 0 up to 3% by moles, preferably from 0.01 to 1% by moles with respect to the total moles of the monomers.
The comonomers having ethylene unsaturation which can be used are both of hydrogenated and fluorinated type; among the hydrogenated ones we can mention ethylene, propylene, acrylic monomers, for example methylmethacrylate, (meth)acrylic acid, butylacrylate, hydroxyethylhexylacrylate, styrene monomers, such as for example styrene. Among the fluorinated comonomers it can be mentioned:
C3-C8 perfluoroolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a C1-C6 perfluoroalkyl;
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
CF2xe2x95x90CFOX (per)fluoro-oxyalkylvinylethers, wherein X is:
a C1-Cl12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per) fluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl; fluorodioxoles, preferably perfluorodioxoles.
Fluorinated comonomers are preferred.
For styrene-based polymers (PS), styrene homopolymers or styrene/acrylonitrile (SAN) polymers, styrene elastomers (SBR), acrylo/butadiene/styrene (ABS) polymers, methacryl/butadiene/styrene (MBS) polymers, etc., are meant. These products are well known in the prior art.
The compositions (nanocomposites) of the invention can be used as such to obtain manufactured articles or coatings having improved mechanical properties, wear resistance.
The nanocomposites of the invention can also be used as additives for styrene-based polymers, due to their compatibility with said polymers. As additives they are used in an amount so that in the final composition there is an amount of PTFE from 0.1% to 10% by weight. When used as additives, the nanocomposites of the invention give an improved processability, for example they allow to operate with a higher extrusion speed, and give an extruded product having improved surface properties (lower roughness).
The core-shell products of the invention allow to obtain manufactured articles with homogeneous properties, both when used alone and as additives. It has been found that there are no segregation phenomena.
The invention compositions are obtainable by a process comprising the preparation of a nanoemulsion containing particles formed by polytetrafluoroethylene polymers having an average diameter from 5 to 100 nm, preferably from 10 to 70 nm, on which a styrene-based polymer is polymerized, the amount of surfactant present in the nanoemulsion before the polymerization of the styrene-based polymer must be such that the surfactant covers the particle surface formed by polytetrafluoroethylene polymers for a percentage from 2 to 100%.
It has been found by the Applicant that if the PTFE nanoemulsion latex is used which comprises all the surfactant necessary for the PTFE polymerization to avoid the PTFE latex coagulation, homogeneous core-shell dispersions are not obtained, since there are also segregated particles formed by styrene-based polymers. This is particularly critical when manufactured articles based on the core-shell nanocomposites of the invention have to be prepared. In this case segregation phenomena would arise which would lead to a final manufactured article having worsened mechanical properties.
More specifically the process for the preparation of the core shell polymer with PTFE-based core and PS-based shell comprises the following steps:
preparation of the PTFE nanoemulsion with particles having the above sizes,
removal of the nanoemulsion surfactant amount until obtaining such an amount as to cover the surface of the PTFE particles for a surface fraction comprised between 2 and 100%,
feeding the so obtained nanoemulsion in a polymerization reactor, maintained under nitrogen flow and under stirring, and addeding:
an amount of water to have a PTFE concentration in the range 5-150 g/l,
styrene, and optionally other comonomers to obtain the mentioned styrene-based polymers,
the polymerization initiator,
then polymerization takes place until obtaining core-shell particles wherein the particle and shell sizes are those above mentioned,
polymer discharge from the reactor.
The polymerization phase of the styrene monomers is carried out according to known methods of the prior art, for example by any temperatures in the range 60xc2x0 C.-80xc2x0 C., initiators which produce radicals at said temperatures, for example alkaline metal or ammonium persulphates.
The preparation of the PTFE nanoemulsion is carried out for example according to the following process:
a) preparation of an aqueous microemulsion of perfluoropolyethers (PFPE) having non reactive end groups or end groups optionally containing 1 or more atoms of H, Cl instead of fluorine;
b) feeding of the microemulsion into the polymerization reactor, in such amount whereby the oil perfluoropolyether phase of the microemulsion is present in a concentration higher than 2 ml per litre of reaction medium, preferably from 2.2 ml up to 50 ml per litre, still more preferably between 3 and 30 ml per litre of reaction medium;
c) feeding of the reaction medium into the polymerization reactor, reactor purge, reactor pressurization with gaseous TFE, optional addition of surfactants, stabilizers, comonomers, transfer agents;
d) addition of the initiator, and optionally during the polymerization of further amounts of surfactants, stabilizers, comonomers, transfer agents;
e) discharge from the reactor of the polymer latex.
The microemulsions used in the process of the present invention are described in U.S. Pat. Nos. 4,864,006 and 4,990,283, herein incorporated by reference, wherein instead of perfluoropolyethers having the mentioned non reactive end groups, also hydrofluoropolyethers having one or both the end groups containing one H atom, or having one or more chlorine atoms at the place of fluorine in the chain end groups, can be used. The molecular weight of perfluoropolyethers (PFPE) which can be used can also be lower than 500, for example 300 as number average molecular weight. The nanoemulsions obtained with the use of PFPE having a low molecular weight, of the order of 350-600, preferably 350-500, can be used advantageously in the applications wherein their quantitative removal is required.
The surfactants which can be used both for preparing the microemulsion and during the polymerization, are (per)fluorinated surfactants known in the prior art and in particular are those described in the cited patents or those having one end group wherein one or more fluorine atoms are substituted by chlorine and/or hydrogen. Among (per)fluorinated surfactants, anionic (per) fluorinated surfactants, having a (per) fuoropolyether or (per) fluorocarbon structure, having for example carboxylic or sulphonic end groups salified with alkaline or alkaline-earth metals, cationic (per) fluorinated surfactants, for example quaternary ammonium salt, and non ionic (per) fluorinated surfactants, can be mentioned. These surfactants can also be used in admixture.
Anionic (per)fluorinated surfactants are preferred and those having salified carboxylic end groups are more preferred.
The molecular weight of the (per)fluorinated chain is in the range 350-1,000, preferably 400-700.
Optionally in the preparation of the PTFE nanoemulsion, iodinated and brominated chain transfer agents can be used. RfI2 can for example be mentioned, wherein Rf is a perfluorocarbon from 4 to 8 carbon atoms.
To obtain the manufactured articles of the invention composition one can add additives commonly used for the styrene-based resins. Fillers, antistatics, antioxidants, plasticizers, impact modifiers, stabilizers, dyes can be mentioned.
The manufactured articles can be also obtained by direct moulding or extrusion starting from the granules obtained by coagulation of the latex. Said articles show high homogeneity and are crack and surface defect free.
The coatings are obtainable by applying on the substrata the compositions of the invention, optionally added with the known additives of the prior art, by spray method or by dipping.
The following Examples illustrate the invention without limiting the scope thereof.